Reduced solar radiation and frequent volcanic activity caused global cooling between the thirteenth and fifteenth centuries, leading to a phenomenon known as the Little Ice Age.

Researchers from the University of Bern and Oeschger Centre for Climate Change Research simulated conditions on CSCS supercomputer Monte Rosa, searching for a process capable of beginning the Little Ice Age.

This research shows the Little Ice Age was primarily triggered by external factors rather than the North Atlantic Oscillation (NAO), which was the previous belief.